Corrugated cardboard is constituted by a multi-layer web product, constituted by at least a smooth layer joined by gluing to a corrugated layer. The smooth layer is usually called “liner”. Normally, the corrugated cardboard has at least two liners, between which and to which at least one corrugated layer is enclosed and glued. The structure of the cardboard may also be more complex, when greater thickness is required. For example, smooth and corrugated layers may be alternated. In this case, the two external layers are constituted by liners, that is they are smooth layers, between which corrugated layers separated by smooth layers are distributed alternately.
Webs of smooth cardboard are used to produce this type of product. In a first machine, called “single facer”, corrugation of a first smooth layer or web is performed by a pair of corrugating cylinders. The corrugated web is joined to a smooth web or liner after applying a glue to the flutes produced on the corrugated web. Examples of machines of this type are described in U.S. Pat. No. 3,527,638; U.S. Pat. No. 5,628,865; U.S. Pat. No. 5,785,802; U.S. Pat. No. 5,415,720.
Joining a multi-layer semi-finished product, constituted by at least a smooth liner and a corrugated layer, to another smooth layer is obtained in machines or devices called “double facers”. Examples of these machines are described in EP-B-0.648.599; EP-A-0.949.064; EP-A-0.949.065; U.S. Pat. No. 5,526,739; U.S. Pat. No. 6,189,445; U.S. Pat. No. 5,837,974; U.S. Pat. No. 5,456,783; U.S. Pat. No. 5,466,329; U.S. Pat. Nos. 5,256,240; 5,005,473; WO-A-9924249; EP-A-0.750.986; EP-A-1.101.599; EP-A-0.839.642; EP-A-0.862.989; EP-A-0.819.054; U.S. Pat. No. 5,836,241; EP-B-0.409.510; U.S. Pat. No. 5,466,329; U.S. Pat. No. 5,561,918.
These devices are constituted, briefly, by a series of heated surfaces or plates, over which the cardboard travels. More precisely, a smooth sheet or “liner” is fed to the device and is glued to a semi-finished product constituted in turn by a smooth sheet previously glued to a corrugated sheet. Several semi-finished products constituted by a smooth sheet and by a corrugated sheet glued together may be fed simultaneously to the device to produce cardboard with several corrugated layers. Previously, glue is applied to the flute tips of the corrugated sheet or layer that is brought into contact with the liner. The cardboard constituted by several layers is then pressed against the heated plates by a series of pressure elements. A flexible pulling element, such as a belt or mat may be provided between these pressure elements and the cardboard.
In “double facer” devices of older conception, the pressure elements were constituted by rollers arranged in series along the longitudinal direction of feed of the cardboard, with their axes of rotation orthogonal to the direction of feed. This type of device had the serious drawback of not allowing even pressure on the cardboard, as the rollers do not adapt to the deformations of the underlying hot plates, which become deformed due to thermal dilations. This results in uneven gluing of the layers of corrugated cardboard produced.
To overcome these drawbacks devices have been produced wherein the rollers are replaced by pressure elements of different arrangement. U.S. Pat. No. 5,456,783 and EP-B-0.623.459 describe a pressure device comprising a series of transverse beams, connected to which are pressure elements or pressure shoes, elastically stressed against the underlying cardboard and fastened to the transverse beams by levers that limit movements in space.
EP-B-0943.423 and U.S. Pat. No. 6,189,445 describe a system similar to the previous one, wherein the pressure elements or shoes, however, are fastened to the transverse beams by a mobile support and springs that allow increased mobility of the shoes. The support rests on the shoes through the springs.
Systems wherein the pressure is exerted by liquid-filled bags or vessels, resting on plates which in turn rest directly on the cardboard have also been devised (see EP-B-0.409.510).
The object of all these systems is to adapt the pressure elements to the deformations of the underlying heated plates.
A further critical aspect in the production of corrugated cardboard is constituted by the possibility of modifying the pressure profile along the direction of feed of the cardboard. For this purpose, devices have been produced with adjustment actuators, distributed along the full length of the pressure system. An example of device of this type is described in U.S. Pat. No. 5,466,329. This type of solutions is particularly complex and costly, also as it is necessary to provide a high number of actuators.
EP-A-0.750.986, U.S. Pat. No. 5,746,010, U.S. Pat. No. 5,853,527 and U.S. Pat. No. 5,832,628 describe a device wherein the pressure on the cardboard is applied by a sort of mat hung at its ends so that the position in space can be adjusted by lifting one of the two ends. In this way the contact surface between the pressure belt and the underlying cardboard is reduced. Nonetheless, the solution offers limited advantages, as this device only allows to increase or decrease the active length of the device, that is the length along which there is pressure between the cardboard and underlying hot plates. On the contrary, it is not possible to modulate and adjust the pressure profile.
“Double facer” devices have a useful width determined by the width of the hot plates, that is by the transverse extension of these plates, orthogonal to the longitudinal direction of cardboard feed, as well as by the width on which the pressure elements act. Corrugated cardboard is produced even in very small batches, differing from one another in the type of material, number of layers and also in width. This means that in many circumstances the device processes cardboard the width of which is below the maximum operating width. When this occurs, there is greater wear on the pressure elements. In fact, in the side bands, where there is no cardboard, these press directly on the underlying hot plates or against the belt or mat disposed between the pressure elements and the cardboard. In this case, the edge areas of the belt (at the level of which there is no cardboard) rub directly on the hot plates and are subject to extremely high thermal stress.
In addition to the problem of wear, direct pressure contact between the belt and hot plates prevents normal lateral release of the humidity contained in the glue, with consequent difficulties in gluing.